The above code derives a 256-bit key using the PBKDF2 key derivation algorithm from the password s3cr3t*c0d3. It uses a random password derivation salt (128-bit). This salt should be stored in the output, together with the ciphertext, because without it the decryption key cannot be derived again and the decryption will be impossible.

The derived key consists of 64 hex digits (32 bytes), which represents a 256-bit integer number. It will be different if you run the above code several times, because a random salt is used every time. If you use the same salt, the same key will be derived.

The ciphertext consists of 38 hex digits (19 bytes, 152 bits). This is the size of the input data, the message Text for encryption.

Note that after AES-CTR encryption the initial vector (IV) should be stored along with the ciphertext, because without it, the decryption will be impossible. The IV should be randomly generated for each AES encryption (not hard-coded) for higher security.

Note also that if you encrypt the same plaintext with the same encryption key several times, the output will be different every time, due to the randomness in the IV. This is intended behavior and it increases the security, e.g. resistance to dictionary attacks.

AES Decryption (CTR Block Mode)

Now let's see how to decrypt a ciphertext using the AES-CTR-256 algorithm. The input consists of ciphertext + encryption key + the IV for the CTR counter. The output is the original plaintext. The code is pretty simple:

Note that the aes object should be initialized again, because the CTR cipher block mode algorithm keeps an internal state that changes over the time.

Note also that the above code cannot detect wrong key, wrong ciphertext or wrong IV. If you use an incorrect key to decrypt the ciphertext, you will get a wrong unreadable text. This is clearly visible by the code below:

Now it is your time to play with the above code example. Try to to encrypt and decrypt different messages, to change the input message, the key size, to hard-code the IV, the key and other parameters, switch to CBC mode, and see how the results change. Enjoy learning by playing.

AES-256-GCM Example

Now, let's give a full example how to use the AES-256-GCM symmetric encryption construction. We shall use a different Python library for AES, called pycryptodome, which supports the the AES-256-GCM construction:

pip install pycryptodome

Next, let's play with the below AES-GCM example in Python, which generates a random encryption key (secret key) and uses it to encrypt a text message, then decrypts it back to the original plaintext message:

The AES-GCM encryption takes as input a message + encryption key and produces as output a set of values: { ciphertext + nonce + authTag }.

The ciphertext is the encrypted message.

The nonce is the randomly generated initial vector (IV) for the GCM construction.

The authTag is the message authentication code (MAC) calculated during the encryption.

The encryption key size generated in the above code is 256 bits (32 bytes) and it configures the AES-GCM cipher as AES-256-GCM. If we change the key size to 128 bits or 192 bits, we shall use AES-128-GCM or AES-192-GCM respectively.

It is visible that the encryption key above is 256 bits (64 hex digits), the ciphertext has the same length as the input message (43 bytes), the IV is 128 bits (32 hex digits) and the authentication tag is 128 bits (32 hex digits). If we change something before the decryption (e.g. the ciphertext of the IV), we will get and exception, because the message integrity will be broken:

The above code encrypts using AES-256-GCM given text message by given text password.

During the encryption, the Scrypt KDF function is used (with some fixed parameters) to derive a secret key from the password. The randomly generated KDF salt for the key derivation is stored together with the encrypted message and will be used during the decryption. Then the input message is AES-encrypted using the secret key and the output consists of ciphertext + IV (random nonce) + authTag. The final output holds these 3 values + the KDF salt.

During the decryption, the Scrypt key derivation (with the same parameters) is used to derive the same secret key from the encryption password, together with the KDF salt (which was generated randomly during the encryption). Then the ciphertext is AES-decrypted using the secret key, the IV (nonce) and the authTag. In case of success, the result is the decrypted original plaintext. In case of error, the authentication tag will fail to authenticate the decryption process and an exception will be thrown.